kidney itself by means of an apparatus known as the onco- 

 meter. With each rise in the blood pressure the kidney swells, 

 with each fall in pressure it contracts, and a tracing can be 

 obtained very similar to an ordinary blood pressure tracing. 

 The glomerulus suspended in its capsule is also influenced by 

 these changes. AVhen the capillaries are engorged with blood 

 the glomerulus fills the capsule, when collapsed there is an 

 ' evident space between the membranes of the glomerulus and 

 the capsule. 



In 1842 Bowman advanced a theory relating to the pro- 

 duction of urine which, while recognizing to some extent cer- 

 tain physical factors, also brought out the view that the urine 

 is a secretion. 



In 1844 Ludwig advanced the theory, sometimes referred 

 to as the mechanical theory, in which only physical processes 

 were involved, i. e., filtration, diffusion and osmosis. It is true 

 that in the capillaries of the glomerulus there is a high resist- 

 ance because of the smaller size of the efferent vessel, there is, 

 therefore, a higher degree of pressure in the glomerulus than 

 in the capsule in which it is suspended. This inequality of 

 pressure is favorable to filtration. As the result of any filtra- 

 tion from the glomerulus some of the water of the blood with 

 a certain proportion of dissolved substances would pass into the 

 beginning of the uriniferous tubule. The effect of such a 

 filtration would render the blood remaining in the glomerulus 

 and second set of capillaries more concentrated, and in the 

 second set of capillaries in connection with the uriniferous 

 tubules the essential elements of an osmometer would be 

 obtained, — an animal membrane formed by the delicate wall 

 of the capillary and the wall of the tubule, upon one side of 

 which there is a dense fluid, the blood, and upon the other a 

 vt^eak saline solution, conditions which are favorable to the 

 processes of osmosis and diffusion. 



In this theory, as a result of the interchange of these 

 fluids, some of the water passes from the tubule to the blood, 

 Tiiaking it less concentrated, and the products of retrogressive 

 metamorphosis, — urea and other organic substances, — pass 

 from the blood to the tubule, forming urine. An objection in 

 connection with the diffusion of urea has been raised in that 



